Monoblock MOSFET amplifier

Finally I got around to write something about this MOSFET amp I have been using ever since 1996. It is based on a Velleman K4010 DIY kit amp. The kit included the pcb, electronic parts, mains transformer (toroid), bridge rectifier and the heatsink. I had some pieces of MDF sawn to make a case, put a handle on the side and made a preamplifier to be able to connect a large range of signal sources, from microphone to pro mixing desk. The preamp used to include a rumble filter, a 25 Hz 4th order highpass Bessel filter, but now it has the opposite: a 85 Hz 4th order lowpass Bessel filter, so it can drive a passive subwoofer. There's also a power level meter, which consists of a 30 LED spot meter.

The picture below shows the top panel removed. In the front are the power switch, the preamp gain knob (a 5-way turn switch), the master volume and the two parallel inputs (for daisy chaining), one below each knob. The bottoms-up PCB is the preamp (don't ask me for a schematic, it was many years ago, and modified a few times since then), and underneith, not visible, is the level meter board containing a U1096B chip. You can see the wires going to the LEDs. Next to that is a small power supply for the fan and level meter, and finally the fan. The fan isn't really necessary and for low volume practise you can switch it off.

Then, in the back, there's the heatsink with the main PCB and the bridge rectifier attached, the main transformer and the power input and speaker outputs (Neutrik XLR and 1/4" jack).

This amplifier has a somewhat unique design. In principle it has a class AB output stage, but it remains in class A until as much as 60 watts. So only during the peaks (it's 155 watts max), the amplifier leaves its class A configuration, making it essentially a class A amplifier.

If you look closely, you can see my tweaks. I tried to make the amp more stable during clipping by trimming the driver opamps' offsets and increasing the coupling capacitor's value. After thorough investigation with an oscilloscope I concluded the unstability was unsolvable. I've removed the tweaks after that.

Click to open a larger view:
Monoblock MOSFET amplifier

This is the rig I use the amp in mostly (see also here):
Small rig

As of 2012, the amp is part of our home theater's sound system. So after 18 years, it's still in use.

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